Coupled alkali-feldspar dissolution and secondary mineral precipitation in batch systems: 1. New experiments at 200 °C and 300 bars

Abstract

Batch reactor experiments were conducted to assess perthitic alkali-feldspar dissolution and secondary mineral formation in an initially acidic fluid (pH = 3.1) at 200 °C and 300 bars. Temporal evolution of fluid chemistry was monitored by major element analysis of in situ fluid samples. Solid reaction products were retrieved from two identical experiments terminated after 5 and 78 days. Scanning electron microscopy revealed dissolution features and significant secondary mineral coverage on feldspar surfaces. Boehmite and kaolinite were identified as secondary minerals by X-ray diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy analysis of alkali-feldspar surfaces before and after reaction showed a trend of increasing Al/Si ratios and decreasing K/Al ratios with reaction progress, consistent with the formation of boehmite and kaolinite.

Saturation indices of feldspars and secondary minerals suggest that albite dissolution occurred throughout the experiments, while K-feldspar exceeded saturation after 216 h of reaction. Reactions proceeded slowly and full equilibrium was not achieved, the relatively high temperature of the experiments notwithstanding. Thus, time series observations indicate continuous supersaturation with respect to boehmite and kaolinite, although the extent of this decreased with reaction progress as the driving force for albite dissolution decreased. The first experimental evidence of metastable co-existence of boehmite, kaolinite and alkali feldspar in the feldspar hydrolysis system is consistent with theoretical models of mineral dissolution/precipitation kinetics where the ratio of the secondary mineral precipitation rate constant to the rate constant of feldspar dissolution is well below unity. This has important implications for modeling the time-dependent evolution of feldspar dissolution and secondary mineral formation in natural systems.

Introduction

Silicate mineral dissolution and secondary mineral precipitation are integrated processes in chemical weathering and hydrothermal alteration of rocks. Numerous experiments have been conducted for measuring silicate mineral dissolution rates (Busenburg and Clemency, 1976, Holdren and Berner, 1979, Chou and Wollast, 1985, Knauss and Wolery, 1986, Nagy et al., 1991, Nagy and Lasaga, 1992, Burch et al., 1993, Gautier et al., 1994, Hellmann, 1994, Oelkers et al., 1994, Hellmann, 1995, Nagy, 1995, Stillings and Brantley, 1995, Brantley and Stillings, 1996). The primary focus of many of these studies, however, was to derive mineral dissolution rates from steady state chemical conditions. In such experiments, silicate minerals, mostly feldspars, are dissolved far from equilibrium and secondary mineral precipitation is avoided by adjusting the chemistry and rate of recirculation of the fluid phase. Results of these experiments have been enormously successful, providing a wealth of data on the rate and mechanism of mineral dissolution processes under a wide range of chemical and physical conditions.

Batch reactor experiments of feldspar hydrolysis, on the other hand, provide a different set of data, which address the broader context of congruency and incongruency, phase relations, mineral metastability, and interconnections between dissolution and precipitation reactions. Tremendous progress in our understanding of feldspar hydrolysis in closed systems has come from the seminal work by Helgeson and co-workers (Helgeson, 1968, Helgeson et al., 1969Helgeson et al., 1970, Helgeson, 1971, Helgeson, 1972, Helgeson, 1974, Helgeson, 1979, Helgeson et al., 1984). While the first feldspar hydrolysis experiments conducted in batch reactors provided valuable information on the mechanism of feldspar dissolution, as summarized in Helgeson (1971) and Petrovic (1976), technological development of experimental design and apparatus has allowed the sampling of fluid co-existing with minerals at a wide range of temperatures and pressures (Seyfried et al., 1987). Furthermore, electron microscopy and surface analytical techniques have advanced significantly, which allow more accurate identification of secondary minerals, even when such phases exist on the nanometer size scale (Penn et al., 2001, Zhu et al., 2006).

Here we report results of alkali-feldspar dissolution experiments in well-mixed batch reactors that were performed at 200 °C and 300 bars in order to examine mineral dissolution and precipitation processes in moderately acidic fluids. Although dissolution reactions of single feldspars have been reported in the literature (Busenburg and Clemency, 1976, Holdren and Berner, 1979, Helgeson et al., 1984, Chou and Wollast, 1985, Knauss and Wolery, 1986, Wollast and Chou, 1992, Gautier et al., 1994, Hellmann, 1994, Oelkers et al., 1994, Hellmann, 1995, Stillings and Brantley, 1995, Brantley and Stillings, 1996, Hellmann and Tisserand, 2006), only a few experimental studies have been performed on feldspars with complex compositions (Morey and Fournier, 1961, Lagache, 1976, Rafal'skiy et al., 1990, Tsuchiya et al., 1995). The combination of time series monitoring of fluid chemistry and mineral analysis (scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electron microprobe analysis (EMPA)) at different reaction stages represents an insightful experimental strategy to assess geochemical controls on the temporal evolution of minerals and coexisting fluids. While this approach is relevant to mass transfer processes in a variety of natural and engineered hydrologic and hydrothermal rock–fluid systems, the experimental data are important in that they form a basis for evaluating a number of theories and hypotheses on the kinetics of water–rock interactions.